The following patent documents are believed to represent the current state of the art:
U.S. Pat. Nos. 4,071,766; 4,720,633; 5,250,808; 5,326,971; 5,362,964; 5,417,211; 4,705,949; 5,945,672; 6,365,898; 6,130,434; 6,025,592; 5,103,102; 4,596,928; 4,880,976; 4,992,662; 4,720,622; 5,406,087; 3,218,459; 3,378,684; 4,037,109; 4,448,311; 4,115,689; 4,587,666; 5,323,441; 5,811,803; 6,452,177; 5,898,261; 4,618,938; 6,072,178; 4,929,041 and 6,114,695.